1. Field of the Invention
The present invention relates to a method and apparatus for controlled material removal from the surface of a substrate over a confined region, and more particularly, to a method and apparatus for removing material from the surface of a substrate by a reactive and excited species created in a plasma generated so that the substrate is independent of the means for generating the plasma. The present invention provides a means for accomplishing controlled, localized material removal for error correction in precision figuring of optical surfaces and thickness profile controlling of films of semiconductor devices while avoiding the effects of substrate geometry on the removal process and a reducing the potential contamination of the surface of the substrate.
2. Description of the Prior Art
Conventional processes for thinning and figuring surfaces and films often employ such methods as mechanical polishing, grinding, sputtering, sand blasting, and chemomechanical processes. Each of these prior art processes usually have substantial processing limitations. For example, chemomechanical processes for thinning semiconductor films is a contact method that leaves contaminants on the surface which cause subsurface damage to the substrate. Also, chemomechanical thinning processes do not allow corrections of the spatial variations within the film thickness.
Plasma assisted chemical etching methods to shape surfaces of substrates are an improvement over the prior art processes such as chemomechanical thinning because such plasma processes do not contact the substrate surface, and, therefore, reduce the potential for subsurface damage. U.S. Pat. No. 4,668,336 discloses a method and apparatus for figuring a surface by plasma assisted chemical transport where the substrate surface is mounted in close proximity to at least one electrode of an rf driven reactor having two parallel plate electrodes. Removal of material from the surface of a substrate is controlled by varying the amount of time an electrode with small surface area spends at a given region wherein the entire surface of the substrate is subjected to the presence of reactive gas which in the presence of an rf field between the two electrodes forms a plasma. Thus, material removal is effected by a plasma which is generated at the surface where material removal is desired. A disadvantage to the invention disclosed therein is that the profile of the surface where the material has been removed may not be precisely controlled.
A related patent application entitled "Method and Apparatus for Confinement of a Plasma Etched Region for Precision Shaping of Surfaces of Substrates and Films", by C. B. Zarowin and L. D. Bollinger, filed on the same date hereof and assigned to the same assignee, discloses a plasma assisted chemical etching device which confines the etching plasma to a local surface area of the substrate surface. The local confinement allows control of the material removal footprint and, thus, the control of the profile of the surface where material has been removed. Because the device disclosed therein is of the type where the plasma excitation occurs by rf discharge directly coupled to the substrate (the local substrate surface is effectively an electrode where the reaction is occurring), underlying surface structure may cause variations in the material removal rate which is a primary cause of "print through" problems.
The "print through" problems associated with local coupled plasma generation can be solved by generating a plasma away from the etching surface to carry out the etching reaction "downstream". When plasma assisted chemical etching is done in a "downstream" mode, long-lived chemically reactive species created in the plasma are carried by the flow "downstream" to the etching reaction site. U.S. Pat. No. 4,431,898 describes a method to generate plasma away from the etching surface for "downstream" stripping of photoresist from an entire wafer. The patent discloses an apparatus which has a plasma chamber inductively coupled to a source of A.C. power wherein the semiconductor devices are etched and an alternative embodiment where the stripping or etching of the substrate takes place "downstream". The downstream method produces a dry chemical removal process over the substrate which eliminates damage to the substrate which could result from energetic species. However, the method disclosed therein fails to provide a means to make precise changes to the profile of the surface where material is removed and, therefore, is not useful for local error correction of a substrate surface.
The present invention incorporates the advantages of decoupled plasma assisted chemical etching with a means to perform local etching so as to correct spatial errors in the substrate surface or correct the thickness profile of a film on a substrate.